Potable (i.e., drinking) water is a necessity to which millions of people throughout the world have limited access. Water is often seen as the most basic and accessible element of life, and seemingly the most plentiful. For every gallon in rivers or lakes, fifty more lie buried in vast aquifers beneath the surface of the earth. There is no standard for how much water a person needs each day, but experts usually put the minimum at 100 liters for adults. Most people drink two or three liters. The rest is typically used for cooking, bathing, and sanitation. Adult Americans consume between four hundred and six hundred liters of water each day.
By 2050, there will be at least nine billion people on the planet, the great majority of them in developing countries. If water were spread evenly across the globe, there might be enough for everyone. But rain often falls in the least desirable places at the most disadvantageous times. For example, some cities in India get fewer than forty days of rain each year—all in less than four months. Somehow, though, the country has to sustain nearly twenty percent of the Earth's population with four percent of its water. China has less water than Canada—and forty times as many people. With wells draining aquifers far faster than they can be replenished by rain, the water table beneath Beijing has fallen nearly two hundred feet in the past twenty years.
More than a billion people lack access to drinking water. Simply providing access to clean water could save two million lives each year. Nearly two billion people rely on wells for their water. There were two million wells in India thirty years ago; today, there are twenty-three million. As the population grows, the freshwater available to each resident dwindles, and people have no choice but to dig deeper. Drill too deep, though, and saltwater and arsenic can begin to seep in.
As cities have grown, many rivers have become unfit to provide untreated water. The amount of fecal bacteria in the Yamuna River, the principal source of water for New Delhi, has increased thousands of times over the past decade. Even in the most prosperous neighborhoods of cities like Delhi and Mumbai, water is available for just a few hours each day—and often is contaminated. India's situation is extreme, but other countries have had similar problems.
Water purification processes are well known and used throughout the world. Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for human consumption. Substances that are removed during the process include parasites (such as Giardia or Cryptosporidium), bacteria, algae, viruses, fungi, minerals (including toxic metals such as lead, copper and arsenic), and man-made chemical pollutants. Schistos omiasis causes anemia or organ infection in humans due to parasitic flukes transmitted through feces-contaminated water. This disease causes much suffering in third world countries. Many such contaminants can be dangerous. Other contaminants are removed to improve the water's smell, taste, and appearance.
It is not possible to tell whether water is safe to drink just by looking at it. Simple procedures such as boiling or the use of a household charcoal filter are not sufficient for treating water from an unknown source. Even natural spring water—considered safe for all practical purposes in the 1800s—must now be tested before determining what kind of treatment is needed. Brackish water is water that has up to 2000-5000 ppm (parts per million) total dissolved solids (TDS). “Mildly” brackish water has a TDS of about 5001000 ppm.
Acceptable drinking water specifications (IS:10500-1191) include the following recommended and “acceptable” levels: a TDS of 500 ppm (up to 2000 ppm, if no other source is available); 0.3 ppm iron (up to 1.0 ppm); 1.0 ppm fluoride (up to 1.5 ppm); 0.05 ppm arsenic; 0.03 ppm aluminum (up to 0.2 ppm); with a pH of 6.5-8.5.
There are many potential sources of water, though none is safe for drinking without prior treatment and purification. The water emerging from some deep groundwater may have fallen as rain many decades or even hundreds of years ago. Soil and rock layers naturally filter the groundwater to a high degree of clarity before it is pumped to the treatment plant. Such water may emerge as springs, artesian springs, or may be extracted from boreholes or wells. Deep groundwater is generally of very high bacteriological quality (i.e., a low concentration of pathogenic bacteria such as Campylobacter or the pathogenic protozoa Cryptosporidium and Giardia) but may be rich in dissolved solids, especially carbonates and sulphates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be present including chloride, and bi-carbonate. There may be a requirement to reduce the iron or manganese content of this water to make it pleasant for drinking and cooking. Disinfection is also required. Where groundwater recharge is practiced, it is equivalent to lowland surface waters for treatment purposes.
Water emerging from shallow groundwater is usually taken from wells or boreholes. The bacteriological quality can be variable depending on the source. A variety of soluble materials may be present including potentially toxic metals such as zinc and copper. Arsenic contamination of groundwater is a serious problem in some areas, notably from shallow wells in Bangladesh and West Bengal in the Ganges Delta. Fluoride is also a potentially dangerous contaminant, potentially leading lead to fluorosis—a serious bone disease.
Typically located in the headwaters of river systems, upland reservoirs are usually sited above any human habitation and may be surrounded by a protective zone to restrict the opportunities for contamination. Bacteria and pathogen levels are usually low, but some bacteria, protozoa or algae will be present. Many upland sources have low pH, which must be adjusted.
Low land surface waters, such as rivers, canals and low land reservoirs, will have a significant bacterial load and may also contain algae, suspended solids and a variety of dissolved constituents. Surface water is contaminated with biological and chemical pollutants and may potentially transmit diseases such as diarrhea, dysentery, typhoid, cholera and hepatitis. It should never be used for drinking without treatment and/or disinfection.
Many processes are available for purification of water, with their use depending on the particular contaminants present in the water. Ultrafiltration membranes are a relatively new development; they use a polymer film with chemically formed microscopic pores that can be used in place of granular media to filter water effectively without coagulants. The type of membrane media determines how much pressure is needed to drive the water through and what sizes of micro-organisms can be filtered out. In ultrafiltration, hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained up to about 0.01 microns in size. This removes bacteria and many viruses, but not salts (ions), while water and low molecular weight solutes pass through the membrane.
Reverse osmosis is the process of pushing a solution through a filter that traps the solute on one side of a reverse osmosis membrane and allows the pure solvent to be obtained from the other side of the membrane. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. A reverse osmosis membrane is semipermeable, meaning it allows the passage of solvent but not of solute, down to a particle size of about 0.0005 microns, which is sufficient to remove viruses and salts (ions).
The membranes used for reverse osmosis have no pores; rather, the separation takes place in a dense polymer layer of only microscopic thickness. In most cases the membrane is designed to allow only water to pass through. The water goes into solution in the polymer of which the membrane is manufactured, and crosses it by diffusion. This process normally requires that a high pressure be exerted on the high concentration side of the membrane, usually 4 to 14 bar (60 to 200 pounds per square inch) for fresh and brackish water, and 40 to 70 bar (600 to 1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
Reverse osmosis membranes are designed to remove dissolved salts from water. While water passes readily through the reverse osmosis membrane, dissolved salt passes through very slowly. Under natural conditions of osmosis, water will diffuse through a semipermeable membrane toward a region of higher salt concentration in order to equalize solution strength on both sides of the membrane. In order to overcome and reverse this osmotic tendency, pressure is applied to feedwater, thereby producing a purified stream.
Salt rejection is a measure of how well a membrane element rejects the passage of dissolved ions. Although a reverse osmosis element may be called upon to reject many different ions, sodium chloride (NaCl) is used as a measurement standard. With few exceptions, reverse osmosis membranes reject divalent ions better than monovalent ions such as sodium and chloride. This NaCl (salt) rejection has been universally accepted as the standard for measuring a membrane element's ionic rejection performance.
Reverse osmosis membranes are also called upon to remove, or at least tolerate, other impurities in feedwater, including organics, silica, and gases. The reverse osmosis process is best known for its use in desalination (removing the salt from sea water to get fresh water) and has been used in this way since the early 1970s.
U.S. standards generally allow reverse osmosis-purified water to be used for drinking. In Europe, however, because leaks or breaks in the filters are possible, thus allowing potential escape of pathogens, water from reverse osmosis filtration may not be used for Human consumption without further treatment steps to guarantee destruction of biological pathogens.
Portable reverse osmosis water processors are sold for personal water purification in the home. These units are gravity powered (they need no water pump), and need no electricity. The pressure of gravity pushes/drains the water though the filters, much like a coffee-maker filter. A filter lasts for about seven years before replacement is needed, although it may need to be replaced as often as every year, depending on usage and the condition of the water. Some travelers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use reverse osmosis water processors. Reverse osmosis systems are also now extensively used by marine aquarium enthusiasts, as the domestic water supply contains substances that are extremely toxic to most species of saltwater fish. In production of bottled mineral water, the water passes through a reverse osmosis water processor to remove pollutants and microorganisms, including the smallest microbe known, archaeobacteria.
Evaluation of long-term reverse osmosis element performance involves consideration of more than salt rejection. Membrane flux, element flow capacity, system pressure requirements, membrane fouling rates, membrane response to cleaning operations and tolerance of cleaning procedures, and the durability of the element all can be important factors in choosing an element. Each can affect the overall productivity of a water treatment system and the capital and operating costs associated with it.
Unfortunately, pressure is needed to operate reverse osmosis. In the U.S. water systems, the water pressure in the pipes provides a sufficient pressure to undergo reverse osmosis. Gravity may also provide sufficient pressure in some cases. There is a serious need in developing countries, however, for a system of water purification that can produce large quantities of water (sufficient for a small village) where there is no power source available to produce the pressure necessary for typical reverse osmosis operation.
Hand pump-operated units for ultrafiltration and reverse osmosis are known, such as those made and sold by Katadyn. None of these, however, combine ultrafiltration and a reverse osmosis membrane unit in a system operated by a hand pump that produces water free of bacteria and substantially free of other contaminants (to bring the water within the IS standards) in amounts sufficient to supply more than a few people.
The manufacturers of reverse osmosis membranes sufficiently large for purification systems for a village water supply specifically limit the use of those filters to a pressure range of 200-800 psi (pounds per square inch), equivalent to approximately 14-56 bar. These pressures are unattainable using a hand pump.